The vapor deposition technique for diamond was started by Derjaguin in 1956 (USSE Inv. Certi. No. 339134, 1958) and Eversole in 1958 (U.S. Pat. Nos. 3,030,187 and 3,030,188). Since then further developments have been made in a study by Matsumoto et al. in 1982 (Jpn. J. Appl. Phys., 21, 1982, L183). In such development, various low pressure vapor deposition methods such as thermal CVD, plasma CVD, ion beam deposition, optical CVD, laser CVD and the like have been studied for synthesizing diamond.
For example, the following is a list of subjects of such a vapor deposition technique for diamond:
(1) Improvement in efficiency for film formation; PA0 (2) Temperature reduction in a synthetic process; PA0 (3) Suppression of damage on a base material; and PA0 (4) Selective growth on a desired region.
Among these items, temperature reduction in a synthetic process and selective growth on a desired region are extremely important subjects for implementing various uses of vapor-deposited diamond.
Techniques of vapor-depositing diamond at lower temperatures, plasma-generating CVD techniques such as dc discharge plasma CVD and microwave plasma CVD should first be considered. In the dc discharge plasma CVD, a diamond film is formed in a range of a relatively low vacuum of 150 to 200 Torr at a temperature of about 500.degree. C. With regard to the microwave plasma CVD, it has been reported that vapor deposition was carried out at a temperature of about 400.degree. C. In the dc discharge plasma CVD, however, an abnormal glow discharge can be carried out only in an extremely limited range on an anode. Therefore, this method is remarkably disadvantageous for selectively growing diamond on a desired region of a base material to be provided with diamond. In the microwave plasma CVD, a region allowing formation of diamond is restricted since it is difficult to stably concentrate the discharge to a desired region of base materials having various three-dimensional shapes. Further, a method employing plasma has a disadvantage in that the surface of a base material is damaged by the plasma. While it is possible to attain a temperature reduction in a process by employing the aforementioned plasma CVD, awaited is a technique for coating a base material which is weaker against heat, such as plastic, for example, with diamond for implementing various uses of vapor-deposited diamond. For diamond coating a plastics material, it is necessary to reduce the temperature of the process below 200.degree. C., more preferably below 100.degree. C.
On the other hand, there has been studied a technique of employing light for decomposing and exciting a raw material gas, in order to further reduce the temperature of the process. More specifically, a CVD technique, employing ultraviolet radiation from an excimer laser, a mercury lamp, a deuterium lamp or a rare gas lamp, and a CVD technique employing synchrotron radiation (SR), for example, as ultraviolet radiation of shorter wavelength have been studied. However, it is difficult to generally carry out the CVD employing SR since an apparatus for emitting synchrotron radiation is extremely high-priced and hard to operate.
Various methods using a CVD employing ultraviolet radiation from an excimer laser or the like have also been studied. For example, Japanese Patent Publication No. 3-51675 (1991) discloses a method of irradiating a hydrocarbon reaction gas with high power ultraviolet radiation for decomposing the reaction gas and depositing carbon atoms liberated from the reaction gas on a heated base material. According to this method, a mixed gas of acetylene and hydrogen (acetylene concentration: 10%) is supplied in a reactor and an Si substrate which is heated to 500.degree. C. is irradiated with a KrF excimer laser beam (wavelength: 248 nm) for carrying out vapor deposition, for example. In the prior art disclosing mere employment of an excimer laser beam etc. for photodecomposition, however, it has been impossible to form high quality diamond having a sufficient intensity of a diamond peak at 1333 cm.sup.-1 in Raman spectrometry. It is believed that this lack of intensity is due to the fact that generally employed ultraviolet radiation such as the excimer laser beam cannot photodecompose H.sub.2, and atomic active hydrogen, which is conceivably an important chemical species for vapor deposition of diamond, is not sufficiently generated.
In the light of the above, a modified method of additionally generating plasma by dc discharge, RF or microwaves has been developed in a CVD technique of applying a laser beam etc. to photodecomposition. According to said modified method, it is possible to generate hydrogen atoms and form diamond of relatively high quality at a relatively low temperature. However, in the modified method it is necessary to generate plasma in an area which is as close as possible to a substrate in consideration of a mean free path. In order to deposit diamond on a three-dimensional region of a base material, therefore, an apparatus of a considerably complicated mechanism is required for uniformly diffusing atomic hydrogen on a base material surface. Further, the employment of the plasma facilitates the deterioration of the base material surface by the plasma and diamond is disadvantageously formed on an unnecessary area by decomposition of a carbon source in the plasma or the quality of the deposit is substantially reduced by an excessive supply of carbon.